Harold Jewett, a senior deputy district attorney for Contra Costa County, would not comment on the scope of the scandal other than to say, "We're following this investigation wherever it leads."

In a search-warrant affidavit filed in court earlier this week, a sheriff's investigator wrote that two of Tanabe's colleagues said he had talked of making drunken-driving arrests intended to ensure that the targets were "dirtied" before future court dates.

Tanabe was connected to three arrests in which he was working with Butler, the affidavit said. In at least one case, Tanabe was suspected of receiving "financial benefits" for making the arrest, the affidavit said.

Tanabe told a colleague that the arrest was a "dirty DUI," the affidavit said.

Tanabe was booked last week on suspicion of possession and transfer of an assault rifle and conspiracy to possess and sell controlled substances. He was released on $260,000 bail and has not been charged with a crime.

Tanabe worked in the sheriff's patrol division and was a familiar figure in Danville, where he was assigned for nearly four years.

"Although Tanabe has resigned, an administrative investigation is continuing," Sheriff David Livingston said in a statement. "In addition, a multi-agency criminal investigation is still under way."